The present disclosure relates to a fixing device and an image forming apparatus including this fixing device, particularly an electromagnetic induction heating type fixing device and an image forming apparatus including this fixing device.
An electromagnetic induction heating type fixing device generates an eddy current in an induction heat generation layer provided in a heating member by a magnetic flux generated in an excitation coil. The fixing device then generates heat in the induction heat generation layer by Joule heat generated by the eddy current to heat the heating member to a predetermined fixing temperature. In this type of the fixing device, since a heat capacity of the induction heat generation layer can be reduced, a warm-up time required for activating the device can be shortened and heat exchanging efficiency can be enhanced. However, when a size of a sheet to be used in fixing process is small, in a sheet passing region in the heating member on which the sheet passes, heat of a surface of the sheet passing region is absorbed by the sheet and a temperature of the sheet passing region becomes low. On the other hand, a sheet not-passing region in the heating member on which the sheet does not pass becomes a high temperature state. In particular, when the sheets are continuously made passed, if the sheet passing region in the heating member remains in the fixing temperature, a temperature of the sheet not-passing region of the heating member is excessively increased, thereby causing failures that temperatures of the heating member and excitation coil exceed a heat resistance limit and that such components are thermal-damaged.
By contrast, some fixing devices solving the above-mentioned failures may be proposed. For example, there is a fixing device provided with a metal sleeve (a heating member), an induction coil (an excitation coil) generating a magnetic flux induction-heating the metal sleeve and a magnetic flux shielding means (a magnetic flux shielding member) inserted between the metal sleeve and induction coil. In this fixing device, the magnetic flux shielding means is configured to move along an axis direction of the metal sleeve. When a small-sized sheet is used in the fixing process, the magnetic flux shielding means is inserted from an end part of the metal sleeve in the axis direction between the metal sleeve and induction coil. According to this, the magnetic flux affecting the sheet not-passing region is cut off and heat generation of the sheet not-passing region by the metal sleeve is restrained.
There is another fixing device in which an excitation coil and a magnetic core are located inside a fixing roller (a heating member) and a magnetic flux shielding member (a magnetism shielding member) enclosing the excitation coil and magnetic core so as to pass between the excitation coil or the magnetic core and the fixing roller. The magnetic flux shielding member includes a flexible base layer (a belt member) and a metal shielding layer arranged in a predetermined area of the base layer and is stretched between a drive shaft and a tension shaft. In this other fixing device, when the small-sized sheet is used in the fixing process, the magnetic flux shielding member is rotated in a circumference direction, and accordingly, the shielding layer is inserted between the fixing roller and excitation coil. According to this, the magnetic flux affecting the sheet not-passing region is cut off and heat generation of the sheet not-passing region by the fixing roller is restrained.
There is a further fixing device provided with a coil part (an excitation coil) generating a magnetic flux induction-heating a fixing belt (a heating member), a magnetic core and a shielding member (a magnetism shielding member) enclosing the excitation coil and magnetic core so as to pass between the excitation coil and fixing belt and shielding the magnetic flux. The shielding member is an endless belt-like member made from a thin film metal and is stretched by a supporting shaft. In the shielding member, an opening part and a covered part (a portion except for the opening part) shielding the magnetic flux are formed. In this further fixing device, when the magnetic flux is shielded, the shielding member is rotated in a circumference direction, and accordingly, the covered part is inserted between the fixing belt and excitation coil. According to this, heat generation of the fixing belt is restrained.